Embodiments of the invention relate generally to MR imaging systems and, more particularly, to an apparatus and method for adjusting alignment of superconductive main coils and superconductive shield coils of the magnet assembly to prevent magnet inhomogeneity and/or structural failure due to the misalignment.
MR imaging systems are known to employ superconducting magnets for a variety of applications, most notably medical diagnostics and procedures. Known superconducting MRI magnet designs include those having a plurality of superconductive main coils, wherein majority of superconductive main coils each carry an identical electric current in the same direction. In some implementations, individual coils within the main coil assembly may carry current in opposite direction to the majority of the main assembly. These superconductive main coils create a static magnetic field within an MRI imaging volume, the MRI imaging volume typically having a spherical shape centered within the magnet's bore, where the object to be imaged (e.g., a human) is placed.
Due to stray magnetic fields that would originate from the magnet if only the superconductive main coils were present, some type of shielding is generally utilized to prevent the high magnetic field created by and surrounding the superconductive main coils from adversely affecting electronic equipment and other objects in the vicinity of the magnet. One type of shielding, known as passive shielding, uses a cylindrical ferrous shield positioned radially about an outer circumference of the superconductive main coils to prevent the stray magnetic field from “leaking” outside of the machine. However, such passive shielding is unsuitable for many MR system applications, as it greatly increases the size and weight of the machine, making it difficult to construct, transport, and implement in some medical facilities. Another type of shielding, known as active shielding, has been found to have greater applicability in modern MRI systems. Active shielding uses a plurality of superconductive shielding coils carrying electrical currents substantially equal to the electrical currents carried in the superconductive main coils, but in an opposite direction. The superconductive shielding coils are positioned radially about an outer circumference of the superconductive main coils, thereby counteracting the high magnetic field created by and surrounding the superconductive main coils to prevent adverse interaction of electrical equipment or other objects with a stray magnetic field.
Known superconducting magnets with active shielding typically encompass a patient bore in an MR system, with a plurality of main coils and a plurality of shield coils affixed to a single former member. The single former member is configured to be contained within the confines of a helium vessel or other cryogenic liquid vessel, which acts to maintain the temperature of the respective coils at an acceptable level. Unfortunately, however, the necessity to construct the single former member to be structurally capable of supporting the strong opposing forces of shield coils and main coils affixed thereto results in an expensive and labor-intensive magnet construction process.
In an effort to reduce material and construction costs, as well as to increase manufacturing efficiency, magnets having two distinct formers to retain the respective main coils and shield coils have been conceived. However, due to possible imperfections during construction and assembly of two distinct formers within the magnet assembly, some amount of longitudinal misalignment between shield coil former and the main coil former is likely, even if the misalignment is only on the order of several millimeters. Such a misalignment, no matter how small, may cause a non-restoring electromagnetic force in the axial direction, thereby causing a shearing effect between the shield coil former and main coil former. This electromagnetic force grows linearly with the misalignment, which in turn generates a self-amplifying positive feedback. As the misalignment increases, the force continues to grow. In addition to the force implications posed by the misalignment, a substantial final misalignment between the main coil former and the shield former coil may also result in a large magnet inhomogeneity.
It would therefore be desirable to have a system and method capable of reducing misalignment and inhomogeneity between main coil assemblies and shield coil assemblies when the main coils and shield coils are retained on separate respective formers in a superconducting magnet assembly.